The appendix (or vermiform appendix; also cecal [or caecal] appendix; also vermix) is a blind-ended tube connected to the cecum, from which it develops embryologically. The cecum is a pouchlike structure of the colon, located at the junction of the small and the large intestines.

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The human's appendix averages 11 cm in length but can range from 2 to 20 cm. The diameter of the appendix is usually between 7 and 8 mm. The longest appendix ever removed measured 26 cm from a patient in Zagreb, Croatia.[1] The opening of the appendix is occasionally guarded by the valve of Gerlach, named after German anatomist Joseph von Gerlach. This valve consists of a semicircular fold of mucous membrane.[2] The appendix is usually located in the right lower quadrant of the abdomen, near the right hip bone. Its position within the abdomen corresponds to a point on the surface known as McBurney's point (see below).

While the base of the appendix is at a fairly constant location, 2 cm below the ileocecal valve, the location of the tip of the appendix can vary from being retrocecal (behind the cecum) (74%)[6] to being in the pelvis to being extraperitoneal.

Possible function of the human vermiform appendix as a "safe house" for beneficial bacteria in the recovery from diarrhea.

Although it has been long accepted that the immune tissue surrounding the appendix and elsewhere in the gut—called gut associated lymphoid tissue—carries out a number of important functions, explanations were lacking for the distinctive shape of the appendix and its apparent lack of specific importance and function as judged by an absence of side effects following appendectomy.[7]

William Parker, Randy Bollinger, and colleagues at Duke University proposed in 2007 that the appendix serves as a haven for useful bacteria when illness flushes those bacteria from the rest of the intestines.[8][9] This proposal is based on a new understanding of how the immune system supports the growth of beneficial intestinal bacteria,[10][11] in combination with many well-known features of the appendix, including its architecture, its location just below the normal one-way flow of food and germs in the large intestine, and its association with copious amounts of immune tissue. Research performed at Winthrop University-Hospital showed that individuals without an appendix were four times more likely to have a recurrence of Clostridium difficile colitis.[12] The appendix, therefore, may act as a "safe house" for commensal ("good") bacteria. This reservoir of gut flora could then serve to repopulate the digestive system following a bout of dysentery or cholera.[13]

The human appendix had previously been proposed to be a vestigial structure, a structure that has lost all or most of its original function. The vermiform appendix has been proposed to be the shrunken remainder of the cecum that was found in a remote ancestor of humans. A 2013 study, however, refutes the idea of an inverse relationship between cecum size and appendix size and presence.[14][clarification needed] Ceca, which occur in the digestive tracts of many extinct herbivores, house mutualistic bacteria that help animals digest the cellulose molecules found in plants.[15]

A possible scenario for the progression from a fully functional cecum to the current human appendix was put forth by Charles Darwin.[17] He suggested that the appendix was used for digesting leaves as primates. It may be a vestigial organ of ancient humans that has degraded to nearly nothing over the course of evolution. The very long cecum of some herbivorous animals, such as found in the horse or the koala, appears to support this theory. The koala's cecum enables it to host bacteria that specifically help to break down cellulose. Human ancestors may have also relied upon this system when they lived on a diet rich in foliage. As people began to eat more easily digested foods, they may have become less reliant on cellulose-rich plants for energy. As the cecum became less necessary for digestion, mutations that were previously deleterious (and would have hindered evolutionary progress) were no longer important, so the mutations have survived. It's suggested that these alleles became more frequent and the cecum continued to shrink. After millions of years, the once-necessary cecum degraded to be the appendix of today.[17]

Appendicitis (or epityphlitis) is a condition characterized by inflammation of the appendix. Pain often begins in the center of the abdomen, corresponding to the appendix's development as part of the embryonic midgut. This pain is typically a dull, poorly localized, visceral pain.[20]

As the inflammation progresses, the pain begins to localize more clearly to the right lower quadrant, as the peritoneum becomes inflamed. This peritoneal inflammation, or peritonitis, results in rebound tenderness (pain upon removal of pressure rather than application of pressure). In particular, it presents at McBurney's point, 1/3 of the way along a line drawn from the anterior superior iliac spine to the umbilicus. Typically, point (skin) pain is not present until the parietal peritoneum is inflamed, as well. Fever and an immune system response are also characteristic of appendicitis.[20]

Appendicitis usually requires removal of the inflamed appendix, either by laparotomy or laparoscopy. Untreated, the appendix may rupture, leading to peritonitis, followed by shock, and, if still untreated, death.[20]

The surgical removal of the vermiform appendix is called an appendectomy, or appendicectomy.[21] This removal is normally performed as an emergency procedure when the patient is suffering from acute appendicitis. In the absence of surgical facilities, intravenousantibiotics are used to delay or avoid the onset of sepsis. In some cases, the appendicitis resolves completely; more often, an inflammatory mass forms around the appendix. This is a relative contraindication to surgery.

The appendix is also used as a means to access the colon in children with paralysed bowels or major rectal sphincter problems. The appendix is brought out to the skin surface and the child/parent can then attach a catheter and easily wash out the colon (via normal defaecation) using an appropriate solution.[23]

Recently... improved understanding of gut immunity has merged with current thinking in biological and medical science, pointing to an apparent function of the mammalian cecal appendix as a safe-house for symbiotic gut microbes, preserving the flora during times of gastrointestinal infection in societies without modern medicine. This function is potentially a selective force for the evolution and maintenance of the appendix. Three morphotypes of cecal-appendices can be described among mammals based primarily on the shape of the cecum: a distinct appendix branching from a rounded or sac-like cecum (as in many primate species), an appendix located at the apex of a long and voluminous cecum (as in the rabbit, greater glider and Cape dune mole rat), and an appendix in the absence of a pronounced cecum (as in the wombat). In addition, long narrow appendix-like structures are found in mammals that either lack an apparent cecum (as in monotremes) or lack a distinct junction between the cecum and appendix- like structure (as in the koala). A cecal appendix has evolved independently at least twice, and apparently represents yet another example of convergence in morphology between Australian marsupials and placentals in the rest of the world. Although the appendix has apparently been lost by numerous species, it has also been maintained for more than 80 million years in at least one clade.[16]

In a more recent paper, the appendix was found to have evolved at least 32 times (and perhaps as many as 38 times) and to have been lost no more than six times.[14] This suggests that the cecal appendix has a selective advantage in many situations and argues strongly against its vestigial nature. This complex evolutionary history of the appendix, along with a great heterogeneity in its evolutionary rate in various taxa, suggests that it is a recurrent trait.[24]

Such a function may be useful in a culture lacking modern sanitation and healthcare practice, where diarrhea may be prevalent. Current epidemiological data on the cause of death in developed countries collected by the World Health Organization in 2001 show that acute diarrhea is now the fourth leading cause of disease-related death in developing countries (data summarized by The Bill and Melinda Gates Foundation). Two of the other leading causes of death are expected to have exerted limited or no selection pressure.[25][26]